I’ve made this little page because I’ve been asked several times how to replace those strange tubes on those chinese amplifiers (amplificatore a valvole), that are so popular nowadays.

For Italian readers: this page has been the source for writing an article for VideoHiFi, a popular free web hifi magazine.

DISCLAIMER:

I don’t encourage tube substitutions and rolling to inexperienced people. If you’re unsure on what you’re doing, please refer to a competent technician, or ask where you bought the amplifier. Remember that working in tube amplfiers can be VERY DANGEROUS due to high voltages involved, EVEN WITH THE AMP UNPLUGGED from the wall outlet.

First, an equivalence table, please note that NOT all china-tube types are listed here, mainly because they don’t have a direct euro-american substitution. I have to thank Jogi for those tables.

I’ve underlined in the text the typical guru’s expressions, search for them when you read an audio review, or articles in guru’s home pages. Needless to say that I’m not encuraging those practices, I’m all against them and I think World will be a better place without those people.

Yes, I’m racist. I think that hi-end gurus should be ri-educated.

1) How to build hi-end audio devices.

It’s very important for the popular internet guru to be alone when designing audio amplifiers. Team work isn’t so appreciated from the hi-end people, they will think you had to make compromises. Instead, hi-end audio is all against compromises. A fuse on the mains line makes the amp safer? Let’s remove it, it’s in series with the signal chain. Do I have to spend money to make my amplifier safer, to make my loudspeakers a bit easier to handle, to make my cables pass CE certification (or other equivalent U.S. counterpart)? Nah, better spend money on critical tuning and optimization.

But how to design them? First of all, all the best in audio has already been designed and built, most of all in the ’20s or ’30s. So no need to spend time on designing things. Just cut and paste togheter something that will reasonably work, and you can call yourself an audio designer.

I already built a Maida-style regulator for my tube phono preamplifier, but I didn’t develope a PCB for it. Now, searching in my hard drive, I found a design I did some time ago, that never saw the light. So I post it here in the hope that someone finds it useful!

This regulator is well suited to tube preamplifiers and similar low current loads. To adapt it to heavier loads, perhaps you should remove (or reduce) R3, remove C1 (with low currents those are useful to obtain higher noise rejection) and increase heatsinking for the power mosfet. In this case however, I figured that a standard heatsink such as this pictured here will be enough.

What more can I add? I had a free evening and put this thing up. Very few parts, popular transistors (you can substitute them with anything low noise, provided you adjust a bit the emitter resistors – BC549C, BC109C, MPSA18, 2SC1815…), single voltage power supply (I suggest using a regulator, maybe a LM317), standard value components.

Everyone with little electronic experience can tweak the circuit to suit their needs or just adapt it to different transistors.
A note: this circuit is the MKII version, the original one suffered from low impedance input that didn’t match well with some cartridges. This one has a good input impedance, ranging from 45k to 60k depending on the actual transistors used.
Obviously the RIAA is perfect ±0.5dB from 20Hz to 20kHz with standard components. And yes, there is also the fourth time constant!

Finally some updates to this website. What do I have been doing up to now? Well, university isn’t easy.A friend of mine asked to design him a simple guitar amplifier, just to practice at home. He’s now building it, I thought it would be good to post here the schematics, in case someone is interested.It’s simple, probably lightweight if carefully built, and could go in a small combo amp with a 8″ or 10″ loudspeaker.It has a “gain” control (R22) and a “volume” control (R12). Here’s the amp schematics:

(click on the image to enlarge)

And this is the power supply schematic:

(click on the image to enlarge)
Do not forget a 1A slow blow fuse in series the primary of the power transformer!!!